A poly(phenylene sulfide) (hereinafter abbreviated to PPS) fiber has high heat resistance, wet heat resistance, chemical resistance and flame retardancy and is characterized in that the fiber can be used in a very harsh environment. A nonwoven fabric composed of a PPS fiber has been used for filters and bag filters for industrial chemicals, battery separators, and the like. In particular, the nonwoven fabric can desirably be used in a harsh environment requiring heat resistance and chemical resistance.
As fundamental properties, PPS has properties of withstanding such harsh environments. On the other hand, when PPS is shaped into a fibrous material and a nonwoven fabric is obtained therefrom, PPS has the problem that it shows poor dimensional stability against heat, and the problem that the fiber or nonwoven fabric shows a significant thermal shrinkage. For example, a PPS fiber nonwoven fabric prepared by melt blowing can be expected to have excellent filtering performance because the constituent fiber is thin. However, the nonwoven fabric has the problem that it has a low tensile strength and it shows poor dimensional stability against heat. Therefore, in order to obtain a PPS fiber nonwoven fabric that can be practically used, it has been necessary, after forming the nonwoven fabric, for the nonwoven fabric to be subjected to treatment for dimension stabilization against heat in the post step, so that the thermal shrinkage is lowered.
Various proposals for means for improving the dimensional stability against heat of a PPS fiber nonwoven fabric have been made.
For example, Japanese Unexamined Patent Publication (Kokai) No. 57-16954 discloses a method comprising treating a filamentary fiber web with needle punch, thermally shrinking the web to develop spiral crimps, and making the crimped web cohere to improve to improve dimensional stability against heat.
Japanese Unexamined Patent Publication (Kokai) No. 1-292161 discloses a method of making a non-crimped filamentary fiber nonmelting at temperatures up to 400° C. by melt bonding 30% or more of the filamentary fiber. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 2005-154919 discloses a method comprising preparing a woven fabric from a PPS fiber, and biaxially stretching the woven fabric at temperatures of a glass transition temperature or more. However, because these methods require a heat-treating step, a step of making nonmelting or a stretching step, they have the problem that the production processes become complicated, and the production efficiencies are lowered so that the products are likely to become costly.
Furthermore, as methods of improving PPS raw materials, methods of improving the dimensional stability against heat of PPS by adding a branched polymer and a copolymer to the PPS are proposed (U.S. Pat. No. 4,458,189 and Japanese Patent Publication No. 2890470, and the like). However, the methods have the problem that the methods require special raw materials.
On the other hand, various methods of improving the dimensional stability against heat of the PPS fiber itself have been proposed. For example, a method of making PPS nonmelting by subjecting the polymer to oxidation treatment (Japanese Unexamined Patent Publication (Kokai) Nos. 63-182413 and 3-104923, and the like) is proposed. However, the method has the problem that the oxidation treatment makes the resultant fiber very brittle, and the problem that use of an oxidizing agent makes the production process complicated.
Moreover, Japanese Unexamined Patent Publication (Kokai) No. 58-31112 describes that the PPS fiber obtained by a high speed spinning technology has a crystallization temperature of less than 120° C. and a melting point of 285° C., and that it is excellent in heat resistance and dimensional stability. It is said that the crystalline portion having a high melting point of the PPS fiber becomes nuclei to improve the heat resistance and that the amorphous portion thereof contributes to the dimensional stability. However, the PPS fiber has the problem that the relationship between a fiber micro-structure and a crystallinity has not become definite, and the problem that the PPS fiber shows a large thermal shrinkage.
As explained above, none of the PPS fibers that have been proposed have been incapable of being used as fibers for spun-bonded nonwoven fabrics that are prepared by thermocompressive bonding fiber webs without further processing.
Therefore, development of a PPS fiber nonwoven fabric that can be stably produced by a simple production method with high productivity and an excellent economic efficiency and that shows an extremely decreased thermal shrinkage is desired.